1. Field of the Invention
The present invention relates to a subsurface safety valve and, more particularly, to a subsurface safety valve of minimized length.
2. Description of Related Art
Subsurface safety valves are used within wellbores to prevent the uncontrolled escape of wellbore fluids, which if not controlled could directly lead to a catastrophic well blowout. Certain styles of safety valves are called flapper type valves because the valve closure member is in the form of a circular disc, as disclosed in U.S. Pat. No. 3,799,258, or in the form of a curved disc, as disclosed in U.S. Pat. No. 4,926,945. These flappers are opened by the application of hydraulic pressure to a piston and cylinder assembly, as is disclosed in U.S. Re. Pat. No. B1 4,161,219, to move a flow tube against the flapper. The flow tube is biased by a helical spring in a direction to close the flapper in the event that hydraulic fluid pressure is reduced or lost.
These types of valves are relatively expensive to manufacture due partly to the overall dimensions of the valve and its internal components. There is a need to reduce the costs of such valves, and one method of reducing the cost is to reduce the valve's overall length, and thereby reduce the raw material and labor requirements. The minimum length of a safety valve is dictated by the distance the flow tube must be moved to open the flapper and to permit the flapper to close. The flow tube's distance of movement, known as its "stroke", is cumulative in that its distance must be repeated throughout the safety valve as space must be provided within the safety valve's housing for the flow tube's displacement in both the open and closed position, the length of the piston in the open and closed position, and the required length of the compressed spring. This results in a minimum length of at least three stroke lengths, plus the height of the compressed spring.
U.S. Pat. No. 4,860,991 discloses a safety valve with minimized length by locating the piston and cylinder assembly parallel with and alongside the power spring, so that the minimum valve length is two stroke lengths. While this safety valve design provides a beneficial length reduction and thereby a cost reduction there is a need for a safety valve with a further reduced length, as well as a minimized outside diameter but still with a longitudinal bore of a maximized internal diameter. The bore's internal diameter is reduced when a safety valve of this style is designed with a reduced outside diameter, because the safety valve's housing does not have sufficient space to accommodate the thickness of the power spring, the thickness of the piston assembly, the thickness of the flapper, the wall thickness of the housing, and still have a bore of a standard internal diameter for a given tubing size.
The length of a safety valve can also be effected when means are incorporated to lock out the valve and establish secondary hydraulic communication. To "lock out" a safety valve is a term well known to those skilled in the art, and is defined as the ability to temporarily or permanently lock the safety valve's flapper in an open position. A safety valve is locked out when the safety valve fails, such as the seals have failed, or during well workover operations. Once a safety valve is locked out, a secondary or wireline retrievable inset valve is sealably set inside of the longitudinal bore of the safety valve, as described in U.S. Pat. No. 4,252,197, or within a hydraulic communication nipple, and the existing hydraulic control line is used to operate the inset valve.
Previous mechanisms to lock out a safety valve and establish the secondary hydraulic communication pathways added additional length to the safety valve and/or increased the mechanical complexity of the safety valve, thereby increasing the cost of the safety valve.